Developing NDE Technologies for Infrastructure Assessment

by Glenn A. Washer

Debris from the collapse of the Silver Bridge on Dec. 15, 1967.

This article provides an overview of the Federal Highway Administration's (FHWA's) program for developing nondestructive evaluation (NDE) technologies for the inspection and evaluation of highway infrastructure. This program is designed to address several key goals in FHWA's National Strategic Plan.

The article discusses: (1) how new laser technology can help attain mobility goals by reducing the number of structurally deficient bridges, (2) new bridge deck evaluation technologies that help achieve both productivity and mobility goals by reducing traffic delays and reducing the cost of maintenance repairs, (3) new technologies that can assist in the evaluation of bridge condition to ensure safety and promote efficient maintenance strategies, (4) an innovative new study that will determine the reliability of existing bridge inspection procedures, and (5) new technologies for characterizing highway building materials.

Background

A bridge inspector performs an inspection of a steel girder as part of the visual inspection study.

The tragic collapse of the Silver Bridge on Dec. 15, 1967, resulted in the deaths of 46 people. This bridge collapse provided a catalyst for the development of the National Bridge Inspection Standards (NBIS), adopted in April 1971. This standard outlines required qualifications of bridge inspectors, defines the scope of bridge inspection programs, and provides for standardized methods of evaluation and appraisal of bridge condition. Since the inception of NBIS, the periodic inspection of highway bridges has relied largely on visual inspection to provide critical information on the condition and safety of our nation's bridges. During this period, advances in NDE technologies have improved the tools available for inspection; however, few technologies have been widely implemented as part of routine inspection procedures.

The emerging use by state transportation agencies of bridge management systems (BMS) to assist in the evaluation of the bridge inventory requires more detailed and quantitative information on the condition of bridges. To make these new BMS tools most effective, increased use of quantitative NDE technologies is required in the bridge inspection process.

The National Bridge Inventory (NBI) contains more than 25 years of information on the bridge population in the United States, and it provides useful information on the needs of the inventory. NBI indicates that more than 104,000 bridges in the United States are rated as structurally deficient.1 This means that the deck, substructure, or superstructure has been rated as poor or worse or that the bridge has been determined to have a low load-carrying capacity. With such a significant number of bridges in poor condition, the development of effective tools for inspecting and evaluating bridges is clearly urgent.

Effective NDE technologies can impact the bridge inventory in two important ways. By detecting deterioration in its early stages, new technologies can help ensure the safety of highway bridges. Second, these technologies can assist in the management of the bridge system by accurately determining maintenance and repair requirements.

Scientists at NDEVC are field-testing the HERMES ground-penetrating radar system, designed to evaluate bridge decks at high speeds.

The goal of FHWA's NDE program is to improve the state of the practice for infrastructure inspection. This is accomplished by determining the reliability of existing NDE technologies and by developing new tools to solve specific problems that are critical to maintaining safety and managing resources for the bridge inventory and other infrastructure resources. To make this happen, FHWA established the Nondestructive Validation Center (NDEVC) at the Turner-Fairbank Highway Research Center in McLean, Va.

NDEVC is focused on bringing innovative, state-of-the-art technologies to bear on the most critical problems affecting the nation's infrastructure. The center works closely with the states and federal land management agencies to provide research and technology innovations that help assess, maintain, and improve the condition of highway infrastructure.

FHWA NDE Validation Center

The objective of NDEVC is to improve the state of the practice for highway bridge inspection. The center is designed to act as a resource for state transportation agencies, industry, and academia concerned with the development and testing of innovative NDE technologies. NDEVC provides state highway agencies with independent evaluation and validation of NDE technologies, develops new NDE technologies, and provides technical assistance to states exploring the use of these advanced technologies.

The center comprises three elements: the NDE laboratories; component specimens, which are sections of bridges containing defects; and field-test bridges.

The NDE laboratories are the nucleus of NDEVC, providing a facility for the development and testing of NDE technologies. The laboratories include a structural loading floor for constructing mock-ups of field conditions, a radiological laboratory used for creating X-ray images of defects, a computed tomography facility for characterizing materials, and an instrumentation laboratory used for manufacturing prototypes and developing new NDE tools.

Figure 1 — Data from the PERES ground-penetrating radar system shows targets buried at four centimeters deep in a concrete bridge deck.

Five decommissioned highway bridges are used to evaluate NDE methods under realistic environmental conditions. Two steel bridges that are open to traffic and fully instrumented are used to test NDE methods associated with live loading, such as new instrumentation for global bridge monitoring. These bridges are critical to evaluating the effect of restricted access, structure geometry, surface conditions, platform stability, and human factors on the application of NDE methods during normal bridge inspections. These test bridges provide NDEVC with a unique ability to evaluate NDE technologies under the same conditions that normal bridge inspections are typically conducted, providing a powerful tool in the evaluation process.

Visual Inspection Study

Normal inspection practices for highway bridges rely almost entirely on visual inspection to evaluate the condition of the bridge. Since NBIS was adopted in 1971, a comprehensive, national study to determine the reliability of the inspection process has not been conducted. To fulfill this need and to provide a baseline for the evaluation of other NDE technologies, NDEVC initiated a study of the visual inspection of highway bridges in 1998.

This study began with a survey of state transportation agencies concerning their practices and policies regarding bridge inspection. Forty-two states responded to the survey, which asked questions about the training and qualifications of inspectors, management of the bridge inspection program, and usage of NDE techniques. Detailed results of this survey will be published in 2000.

Figure 2 — Deflection of a bridge, as measured by a scanning laser system, shows the load distribution of a truck located along the shoulder of the roadway.

A performance evaluation was conducted to determine the reliability of bridge inspections. This part of the study had teams of bridge inspectors from around the country perform inspections on NDEVC's test bridges. Twenty-five states sent a team of two inspectors to participate in the study. As part of the study, the inspectors were required to inspect seven highway bridges of varying design and condition. Ten different inspection scenarios were used to evaluate the various approaches used by state transportation agencies to inspect bridges. Human factors, such as training, experience, and attitude, were evaluated through a series of questionnaires administered by the NDEVC staff prior to and during the performance of the inspections.

This portion of the study was completed in October 1999, and the detailed results will be available in 2000.

New Technologies for Bridge Assessment

HERMES Ground-Penetrating Radar System

NBI indicates that there are almost 298 million square meters (more than 3.2 billion square feet) of bridge deck in the United States. The majority of these decks consist of reinforced concrete that provides the driving surface for the bridge. The service life of a deck can be much shorter than the substructure and superstructure of a bridge, and estimates indicate that FHWA alone is currently investing as much as $1 billion annually for deck rehabilitation.2

Bridge decks deteriorate due to corrosion of reinforcing steel and the resulting delaminations and spalling that can make a deck structurally deficient. The ability to detect this deterioration in its early stages is critical in directing repairs to the most at-risk bridges and will help optimize the use of limited funds.

Currently available methods for evaluating bridge decks include inspecting the deck condition visually, sounding a bare deck with a chain or hammer, measuring the half-cell potential of the deck, and taking cores. All these methods may require lane closure and have limited ability to determine the internal condition of the deck over the entire deck area. In addition, these methods are not effective in accurately determining the exact location and extent of delaminations in a bridge deck, and they are difficult to apply rapidly to a large number of bridge decks.

Other technologies, such as the infrared thermography and existing ground-penetrating radar (GPR) systems, are also used for the evaluation of bridge decks. These technologies have not satisfied the need for rapid, quantitative bridge deck assessment. Infrared thermography is limited by environmental conditions and has difficulty evaluating decks with asphalt overlays. Existing GPR systems require significant expert analysis to effectively evaluate deck condition, and they have had difficulty providing fast and reliable results that satisfy the needs of state highway agencies.

To address these needs, FHWA has funded the development of HERMES (High-Speed Electromagnetic Roadway Measurement and Evaluation System) by the Lawrence Livermore National Laboratories (LLNL). The goal of the HERMES project is to develop a GPR system that can reliably detect, quantify, and image delaminations in bridge decks. The system is designed to operate at normal highway speeds, eliminating the need for lane closure.

The HERMES system includes a computer workstation and storage device, survey wheel, control electronics, and an array of 64 antenna modules or transceivers mounted in a towable trailer.

The most unique design feature of HERMES is the antenna array. The arrangement of the transceivers gives samples across a two-meter width of the deck at three-centimeter intervals at a speed of about 32 kilometers per hour. At almost 100 kilometers per hour, the intervals are six centimeters. The density of data enables synthetic aperture radar techniques to be used in the processing of the data, and two- and three-dimensional images can be produced. HERMES uses ultra-wide-band microwave sources developed by LLNL that produce signals with a frequency of 0.5 to 5 GHz.

The prototype system was delivered to FHWA in October 1998. A field testing of this system is currently being conducted in cooperation with state transportation agencies. The field-testing program will fully evaluate the prototype system and identify required improvements for a second-generation system.

At NDEVC, engineers from state departments of transportation are trained on the use of advanced NDE technologies.

As part of the HERMES project, a single-antenna scanning device capable of performing high-resolution scanning was constructed by LLNL. Known as PERES (Precision Electromagnetic Roadway Evaluation System), this system is used for laboratory evaluation of radar performance and for the development of image-processing algorithms. The system has been used at NDEVC for developing algorithms and to evaluate potential uses for this innovative technology.

Laser Bridge-Deflection Measurements

Of the approximately 104,000 structurally deficient bridges, more than 21,000 bridges are classified this way due solely to a low structural appraisal rating -- that is, the bridges have a low load-carrying capacity.2 However, this load-carrying capacity is normally determined by theoretical calculation, it may not accurately reflect the true capacity of the bridge.

This has led to the development of guidelines for load-rating bridges experimentally, a process that can be expensive and time-consuming. Technologies that can reduce the time and cost associated with load-rating structures are critical to improving this process. Through the effective use of accurate load-rating procedures, the actual load-carrying capacity of a bridge can be determined, reducing the number of bridges classified as structurally deficient and increasing the accuracy of load posting.

Applications for a laser bridge-deflection system have been developed that can reduce the cost of the load-ratings. The laser system, developed under contract for FHWA, uses a frequency-modulated laser to measure bridge deflection from a range of up to 30 meters. A computer-controlled scanning system controls the laser and allows the system to scan a large area of a structure. Measurement resolutions of less than one millimeter are possible with the system, and no special target or surface preparation is required.

NDEVC has developed a series of applications for this technology for making measurements of deflections for large structures. Figure 2 demonstrates one such application. This figure shows the deflected shape of all seven beams of a bridge loaded by a truck in the breakdown lane. The measurement of all seven girders s obtained from a single location under the bridge without the need to mount targets on the structure. This helps reduce the cost of bridge load-rating, and it provides more information than is typically available from traditional instrumentation. With a range of 30 meters, the laser system is also capable of measuring deflections of bridges crossing above open traffic lanes, eliminating the need for lane closure.

Other applications of this technology that have been developed at NDEVC include deflection measurements of large bridge piers under load and measuring out-of-plane displacements for steel plate girders.

Stress-Measurement Technologies

In the evaluation of highway bridges, it is critical to determine the distribution of loading and to evaluate the stress levels in load-carrying members of the bridge. This is typically accomplished through the use of foil strain gauges. However, these gauges require some surface preparation to install and are unable to measure the distribution of dead load within the structure. NDEVC is currently developing instrumentation and applications for the ultrasonic measurement of stress in bridge members.

The first method being developed involves the use of ultrasonic birefringence to evaluate the state of stress in a steel bridge member. A rotating transducer that launches polarized shear waves (which are confined to one plane) is used to measure the magnitude of principal stresses. Applications of this technology for the evaluation of stress in hanger connection plates have been previously developed.3 Current studies are focused on evaluating the stress level in the main members and lateral bracing systems.

Also being developed are methods for evaluating the stress level in high-strength steel strand used in pre-stressed and post-tensioned concrete applications. Strands are used in these applications to apply compressive loading in the tensile area of bridge beams, increasing the load-carrying capacity of the member. Loss of pre-stressing force in these strands can reduce that load-carrying capacity. The ultrasonic method being developed measures guided wave velocity to determine the force carried in the strands.

NDEVC was also involved with the assessment of the Barkhausen Noise Analysis for determining the stress carried in a structural member, and the center assisted the state of Virginia by providing training in this and other technologies.

Radiography

NDEVC has a nuclear instrumentation laboratory, which is used to develop and evaluate nuclear NDE techniques for highway applications.

The nucleus of the laboratory is a state-of-the-art X-ray computed tomography (CT) system. The CT imaging system consists of dual-focus 420-kilovolt and microfocus 160-kilovolt continuous X-ray sources. The system can benefit many industrial and scientific applications, including materials research, nondestructive testing, core-sample characterization, weld inspection, and failure analysis.

Projects carried out at the laboratory include the evaluation of available radiographic systems for the detection of broken wires in cable-stayed bridges, imaging of post-tensioning strands in concrete beams, and the detection of voids in the grouted post-tensioning ducts.

Projects were conducted to evaluate the use of tomographic imaging for the evaluation of air entrainment and air-void distribution in concrete cores taken from highway bridges. A three-dimensional rendering of a concrete core is shown in figure 3. Imaging of crack propagation caused by chemical attack in concrete cores was also demonstrated.

Another project conducted in the laboratory was the design, fabrication, and evaluation of a portable system for nondestructive determination of chloride concentration in reinforced concrete structures.4 This system identifies areas susceptible to accelerated corrosion.

Epithermal neutron detectors for nondestructive measurement of concrete hydration are also being developed to monitor concrete-curing processes.5

Fatigue-Crack Detection

Since the time of the Silver Bridge collapse, FHWA has had an ongoing interest in the development of more effective tools for the detection of fatigue cracks in steel bridges. Recent work at NDEVC has focused on developing methods for the application of electromagnetic crack-detection systems. This has included studies of the eddy current method and alternating current field measurement for detecting cracking in the area of welded connections. The primary advantage of these technologies is the ability to "see" through paint with minimal surface preparation required.

These innovative crack-detection technologies have been fieldtested in locations around the country to provide technical assistance to state highway agencies exploring new methods for detecting cracks. Field tests were conducted for agencies in Alaska, the District of Columbia, Georgia, and Virginia and for the Eastern Federal Lands Highway Office.

FHWA also funded the development of a coating-tolerant, forced-diffusion thermography system for the detection of cracks in steel structures. The goal of the system being developed is to provide a bridge inspector with a full field representation of critical details with characteristic patterns indicating the presence of a crack. The project, funded under the Small Business Innovation Research (SBIR) Program, is scheduled to be completed later this year. Field testing at NDEVC will be used to determine the capability of the system.

Partnerships

NDEVC has developed partnerships with state highway agencies, other government agencies, and other teams at the Turner-Fairbank Highway Research Center (TFHRC) to develop NDE technologies and improve the state of the practice.

HERMES II Project

The HERMES system described earlier in this article has garnered attention from many sources. Caltrans (California Department of Transportation) is convinced that this new technology has the potential to provide a cost-savings tool that can change the way bridge decks are evaluated. However, further development of this prototype system is required, and the participation of state highway agencies is essential. Caltrans has taken the lead in sponsoring a pooled-fund study to develop a comprehensive workplan for the development of a second-generation prototype.

As part of that effort, NDEVC has moved much of its experimental test program to states across the country. Field tests conducted with the cooperation of state highway agencies in Colorado, Minnesota, Missouri, New Jersey, Pennsylvania, Tennessee, and Virginia improved the evaluation process and allowed more than 200 state and local transportation officials to witness the system in action.

At a meeting of the participating states in January 2000, the groundwork for the development of the next generation of HERMES technology will be laid out.

Box-Beam Testing

NDEVC has been leading a cooperative study with the state of New York to perform full-scale structural testing of five pre-stressed box beams that were removed from a bridge in upstate New York. The testing is determining the remaining strength of the girders and evaluating the potential of composite wrapping systems to strengthen the girders back to their original load-carrying capacity.

Other teams at TFHRC are playing key roles in the project. The Geotechnical Team constructed full-scale abutments using innovative construction techniques, and the High-Performance Materials Team is applying their expertise in full-scale structural testing to assist in the study. At the conclusion of the study, a test bridge constructed from three of the girders will be left in place for the future development of NDE systems that evaluate the condition of pre-stressed concrete beams.

Teaming

NDEVC is providing assistance and technical expertise to enhance the efforts of other programs at TFHRC. In conjunction with the Asphalt Team, for example, new instrumentation is being developed to calibrate gyratory compactors that are used to characterize asphalt pavements.

The previously mentioned laser system is being used to measure the deflections of a full-scale curved-girder bridge being tested in the Structures Laboratory at TFHRC. The unique capabilities of the laser system, coupled with processing algorithms developed at NDEVC, are being used to measure the out-of-plane distortion of the curved girders during testing. The laser system presents a full-field view of the deflections during the testing, allowing engineers to detect and quantify buckling at the early stages of failure.

NDEVC has also actively assisted the Federal Lands Highway Program by providing technical expertise to bridge inspectors who ensure the safety of bridges in their jurisdictions. The assistance has included the use of thermography systems to evaluate the quality of new bridge decks, demonstrating crack-detection methods for steel bridges and training inspectors on the use of instrumentation for load-rating bridges.

Teaming with LLNL, NDEVC demonstrated the use of a variety of NDE techniques for evaluating bridge pins. This study included the evaluation of cracks in pins by ultrasonic and radiographic methods. LLNL imaged the cracked pins using computed tomography; NDEVC imaged the cracked pins using ultrasonic and radiographic methods.

NDEVC provides technical assistance to state transportation agencies through its active field-testing program, which brings innovative technologies and prototype systems to the field to be evaluated. The center has also been active in providing training and technical expertise to the states, using a unique team of technical experts to provide a powerful resource. The staff of the center includes experts in the area of ultrasonics, radiography, optics and lasers, instrumentation, crack detection, wireless data acquisition, bridge inspection methods, structural engineering, and ground-penetrating radar.

Conclusion

The development of new NDE technologies for the inspection of highways and highway bridges will assist FHWA in attaining its stategic goals by helping to effectively manage the highway system in the 21st century. While many NDE tools are available or are being developed for highway application, few have been widely accepted for use in normal bridge inspections. The FHWA NDE Validation Center, through the development and evaluation of NDE technologies, will hopefully broaden the use of NDE and improve the state of the practice for highway bridge inspection.

Glenn A. Washer is the program manager of the Federal Highway Administration's Nondestructive Evaluation Validation Center at the Turner-Fairbank Highway Research Center in McLean, Va. He has a bachelor's degree in civil engineering from Worcester Polytechnic Institute and a master's degree in civil engineering from the University of Maryland. He is currently a doctoral candidate at The Johns Hopkins University's Center for Nondestructive Evaluation. Washer is a licensed professional engineer in Virginia.